Effect of Inflation of a Pressure Suit on Pulmonary Diffusing Capacity in Man

¹ From the Departments of Physiology and Pharmacology, Graduate School of Medicine, University of Pennsylvania, Philadelphia, and the Aviation Medical Acceleration Laboratory, Naval Air Development Center, Johnsville, Pennsylvania

Previous work has shown that inflation around the lower half of the body of a tightly fitting pneumatic suit to a pressure of 75 mm Hg produces an acute increase in pulmonary arterial and wedge pressures of about 25 mm Hg in normal subjects. Effects of this procedure upon the pulmonary capillary bed were investigated by measuring the diffusing capacity of the lung for CO (Dl) at different alveolar O₂ tensions from below 100 mm Hg to above 600 mm Hg in four healthy subjects using a 10-second breath holding technique. Measurements made when the suit was inflated were compared with control measurements made immediately before and/or after with the suit deflated. In none of 11 series of experiments was a significant change in mean Dl produced by inflatino of the pneumatic suit. The true diffusing capacity of the pulmonary membrane for CO (Dm) and the volume of the blood in the pulmonary capillaries (Vc) were also calculated from the value of Dl at different alveolar O₂ tensions. Dm fell following suit inflation in two subjects and rose in two others, while Vc fell in one subject, rose in two subjects and was unchanged in one subject. These changes were probably not significant. Inflation of the suit produced gas ‘trapping’ in the lung. While it is possible that this gas ‘trapping’ produced an underestimate of Dl during suit inflation, one would have expected that, if the procedure had either markedly dilated patent pulmonary capillaries or opened capillaries previously closed, a significant increase in Dl, Dm or Vc would have been observed.